Certain medical or biomedical devices are based on Cr:Er:YSGG or Er:YAG lasers operating in free-running oscillation regime near the wavelength of water absorption −2.8-2.9 μm. Lasers emitting in the 3 μm wavelength region are needed in the medical field as surgical tools. This use of a laser as a laser scalpel or drill is due to the absorption of water in this spectral region. To effectively use such a laser, it must have high energy as well as short pulses that can be provided by Q-switching of Er laser cavities.
The temporal output of the current lasers is characterized by multiple spikes of ˜1 μs pulse duration spreading irregularly over the flashlamp discharge pulse of approximately 100-200 μs. The drawback of irregular character of the spikes is that the spikes with energy below the threshold of teeth ablation deposit their light energy towards teeth heating, resulting in painful sensations that might appear in the teeth of the patients.
To eliminate this problem and the need of anesthesia during treatment it is proposed to utilize passive Q-switched regime of Cr:Er:YSGG operation with a much shorter (<150 ns) but regular multiple pulses each with energy above the threshold of ablation to eliminate pain sensations while preserving cutting efficiency of the dental hard tissue.
The simplest way to obtain the required regime of ns multiple laser pulses with high peak powers in a cost-effective, compact and reliable all-solid-state laser system consists in laser cavity passive Q-switching by inserting a saturable absorber inside the Cr:Er:YSGG resonator. However, commercial passive solid-state Q-switches for the 3 μm spectral range are not currently available. A 2.94 μm Er:YAG laser was Q-switched using a rotating, mirror as reported by Bagdasarov, Danilov et al, electro-optic Q-switch as reported by Bagdasarov, Zhekov et al, and a passive water and ethanol Q-switch as reported by Vodopyanov. Successful realization of the 1.3-2.1 μm laser cavities passive Q-switching with the use of Cr doped ZnSe and ZnS crystals was demonstrated by several research groups. However, the use of Fe doped chalcogenides for the passive Q-switching of laser cavities at longer 2.4-3.4 μm spectral range was not evident and trivial due to a strong non-radiative quenching of the excitation in these materials at room temperature. To characterize the effectiveness of Fe2+:ZnSe as a saturable absorber in the Mid IR Spectral region and as a potential gain medium, the cross-section of absorption versus wavelengths must be measured. As one can see from FIG. 1 the absorption cross section of Fe2+ ion in the ZnSe crystal measured at λ=2.94 μm is ˜9.5×10−19 cm2, which is approximately 35 times higher than the cross section for the laser transition of the Er3+ ion in yttrium-aluminum garnet. The combination of a high value of saturation cross-section, small saturation energy with good opto-mechanical and physical characteristics of the ZnSe host (damage threshold—2 J/Cm2, Knoop Hardness 1.20 kg/mm2, thermal conductivity 18 W/mK, dn/dT=70×10−6 K−1) make Fe2+:ZnSe crystal a promising material for passive Q-switching of mid-infrared laser cavities.
A significant problem is that growth of Fe doped ZnSe crystals is not trivial. Bulk Fe2+:ZnSe crystals can be obtained from melt or vapor growing techniques by including the dopant in the starting charge. Under atmospheric pressure ZnSe is sublimed at a temperature higher than about 1100° C. before melting. It is therefore for melt growth, in addition to high temperature (1525° C.), necessary to apply high pressure, up to 75×10−5 Pa [6]. This inconvenience of the ZnSe high temperature melt growth might be accompanied by uncontrolled contamination inducing undesired absorptions. On the other hand, the control of the amount of Fe2+, ions incorporated in the crystal is difficult using vapor growth technique. Hence, utilization of other more cost effective methods of Fe2+:ZnSe fabrication is of interest.